I did a forum search and was overwhelmed by the amount of info. Where do I begin? Sorry, bit of a newbie here...

My car normally runs at 85-90 degrees all the time. The other day I'm sitting in traffic on a hot day, and my temp is climbing to like 110-115 degrees. I get home and open the hood with the engine still running, and I see that neither of the fans are spinning. The temperature goes back down if I can get up to speed, but when sitting in traffic, it quickly goes back up.

What is the first thing to suspect? Is it the thermostat that controls the fan? Or is it something in the fan clutch that activates it at the right temperature? The fan should be spinning if the temp is that hot, right?

I do have a greasecar kit installed, but that has been in place for months now, so I don't that is related to the problem.

Where do I begin troubleshooting?

Thanks!

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1987 Mercedes 300D ~200K (Greasecar & Biodiesel)
1993 Ford F-250 7.3 IDI diesel 165K (Biodiesel)
1996 Thomas/International Bus with DT466 engine

in your coolant resevor. then check your thermastat

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83' 300SD Turbo, intercooled,
71' Porsche 911 T Targa.
85.5' Porsche 944 euro
87' Fiero cp.
Lamborghini Diablo replica V8 4spd.

Kinda hard to diagnose but I'd start with the clutch fan. The next time the gauge reads over 110*C, pop the hood and stop the engine using the stop lever on the IP. The clutch fan should stop within a blade or two. If several blades pass or it freewheels, you've found a culprit if not the culprit.

Do a search on the 3-pin thermo switch on the upper radiator hose fitting. It looks like a thermostat housing but that ain't where the thermostat is in your engine. Anyway, I think that's the switch that tells the auxiliary fan to turn on a 115*C or some such setpoint. Do a search to confirm which thermoswitch controls the fan. You can ground one of those pins to test the fan.

If this occured with the AC running then do a search on which of the receiver/dryer pressure switches triggers the fan.

Sixto
93 300SD

If you want a temporary fix until you figure out the root cause, you can jump the two terminals for the aux fan at the temp sensor with a short piece of wire and it will keep the car cool, even if the radiator fan isn't working. That's assuming that your aux fan isn't shot. Also check the two fuses for the aux fan, they may be shot too.

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87 300D - Running on Veggie oil; 260,000 plus miles; Original #14 head

In addition to the clutch fan, many of these old vehicles have issues with airflow through the radiator and condenser.

My SD has now taken on a dose of the "100's". Even in 50 degree weather, it can creep up toward 100C. in low airflow conditions.

The clutch fan is working correctly so, I'm quite sure that the passages through the condenser and the radiator are partially clogged with dust and debris. If you take a look at the front of the condenser, you'll find it unbelievable that any airflow whatsoever gets through to the radiator.

So, to remedy this situation, you'll need to remove the radiator from the vehicle and thoroughly clean the fins with condenser cleaner and compressed air or a low powered pressure washer. It's a long tedius job to get all the debris removed. But, hold the radiator up to the sun and peer through it. You'll see the task at hand.

The condenser needs a similar treatment, however, it's more difficult because you can't remove it from the vehicle. I have not done the compressor, personally, but, I'm fairly sure that it's due for the same treatment.

With restricted airflow through the radiator, will the fan clutch get hot enough to engage?

Sixto
93 300SD